1. Field of the Invention
The present invention relates to a brake system of a vehicle, and more particularly, to a brake control device for a vehicle brake system which operates with motor powered pump in response to a depression of the brake pedal by a driver.
2. Description of the Prior Art
An art of powered brake system of vehicles such as automobiles is already known, in which the wheel cylinders of a vehicle are selectively supplied with a brake fluid pressurized by a motor powered pump under a control of electric control means which calculate and determine the pressure of the brake fluid to be supplied to each selected one or ones of the wheel cylinders based upon a depression of the brake pedal by a driver so that a more powerful and more ingeniously controlled braking is applied to the wheels than the braking simply dependent upon a compression of the brake fluid in the master cylinder effected by a depression of the brake pedal by a driver.
An example of such a powered brake system is shown in, for example, Japanese Patent Laid-open Publication 9-30394 based upon German Patent Application 19526659.5 according to which a target deceleration of a vehicle is calculated based upon a pressure of the master cylinder and its change rate, whereby the braking force is controlled to attain a deceleration of the vehicle conforming to the target deceleration. It will be expected that such a control of the braking force based upon the master cylinder pressure and its change rate provides a better braking performance than the conventional hydraulic brake system dependent only upon the master cylinder pressure generated by a depression of the brake pedal.
However, the increase of the master cylinder pressure according to a depression of the brake pedal generally shows such a performance as shown in FIG. 7 that the master cylinder pressure starts to increase only after the brake pedal depression force increases beyond a certain initial idle region, thereby causing an insensible initial region in which the driver's intention for a deceleration of the vehicle is not appropriately read out from the master cylinder pressure.
On the other hand, there is generally such a relationship between the stroke and the force of depression of the brake pedal as shown in FIG. 8, in which, although there is a certain initial idle stroke which can proceed with no reaction of pedal depression, if the initial idle stroke is provisionally taken into consideration, there is no region of initial insensibleness in reading out the driver's intention for a deceleration of the vehicle from the stroke of depression of the brake pedal. Indeed, in driving vehicles, mostly cars, drivers will often step on the brake pedal for a precaution of braking within a depth which corresponds to the intercept of the performance curve of FIG. 8 along the ordinate, generating no reaction force of depression. Although the amount of such a precautionary depression itself is irregular and indefinite, it does not, in any event, affect the running of the vehicle at all, and if the stroke of depression proceeds so far as to cause any pedal depression force indicating an actual intention of the driver for a deceleration of the vehicle, the driver's intention for the deceleration of the vehicle is immediately readable from the stroke with no initial insensibleness.
Further, since the curve of pedal depression stroke vs. pedal depression force taken respectively along an ordinate and an abscissa of rectangular coordinates generally shows such a convex shape as shown in FIG. 8 having a relatively steep slope in the initial region of the depression, the driver's intention for a deceleration of the vehicle is detectable at a high precision in a low braking region by checking the pedal depression stroke, although the precision swiftly substantially lowers when the pedal depression increases.
In consideration of such particular characteristics as described above of the relationship between the master cylinder pressure and the depression force of the brake pedal and the relationship between the depression stroke and the depression force of the brake pedal, the present inventor has proposed, together with his college, an invention with regard to a brake control device for a powered brake system of a vehicle by U.S. patent application Ser. No. 09/263,226, which estimates a braking force to be applied to the wheels of the vehicle in response to the operation of the brake pedal by a driver in such a manner that the inefficiency due to the initial insensibleness of the master cylinder pressure to the brake depression force is obviated, while the high sensitivity of the brake pedal depression stroke to the brake depression force in the initial stage of the brake depression is effectively utilized.
Indeed, in more detail, the above-mentioned formerly proposed brake control device adapted to be incorporation into a vehicle having wheels and a brake system for the wheels including wheel cylinders for applying a braking force to each corresponding one of the wheels according to a supply of a pressurized brake fluid thereto, a brake pedal adapted to be depressed by a driver, a master cylinder adapted to compress the brake fluid according to a depression of the brake pedal by the driver, a motor powered pump for pressurizing the brake fluid, and control means including on-off valves for controlling supply of the brake fluid pressurized by the master cylinder and the brake fluid pressurized by the pump to the wheel cylinders, comprises:
means for detecting a stroke of the depression of the brake pedal, PA1 means for detecting a pressure of the brake fluid compressed by the master cylinder, and PA1 means for determining a pressure of the brake fluid supplied to a selected one or ones of the wheel cylinders under a control of the control means so as to be increased along with increase of the depression stroke of the brake pedal as well as increase of the master cylinder pressure by laying first and second weights on the increase of the brake pedal depression stroke and the increase of the master cylinder pressure, respectively, the first weight being decreased while the second weight being increased relative to one another along with increase of the total target deceleration. PA1 means for detecting a stroke of the depression of the brake pedal, PA1 means for detecting the brake pedal to be depressed at least for a predetermined initial stroke, PA1 means for detecting a pressure value of the master cylinder pressure, PA1 first judgment means for judging the brake depression stroke detected by the brake pedal depression stroke detection means to increase beyond a predetermined threshold value smaller than the initial stroke predetermined for the initial brake pedal stroke detection means as a first positive braking intention of the driver, PA1 second judgment means for judging the brake pedal to be depressed at least for the initial stroke as a second positive braking intention of the driver, PA1 means for determining the brake system to operate such that the brake fluid pressurized by the motor powered pump is supplied to at least one selected from the wheel cylinders under a control of the control means when the first judgment means judge the first positive braking intention of the driver, while the second judgment means judge the second positive braking intention of the driver, PA1 wherein the control means control a pressure of the brake fluid supplied to the selected one of the wheel cylinders such that the vehicle is more decelerated along with increase of the depression stroke of the brake pedal and increase of the master cylinder pressure by laying first and second weights on the increase of the depression stroke and the increase of the master cylinder pressure, respectively, the first weight being decreased while the second weight being increased relative to one another along with increase of the braking applied to the vehicle. PA1 third judgment means for judging the master cylinder pressure to increase beyond a threshold value predetermined therefore as a third positive braking intention of the driver, PA1 means for storing an initial value of the stroke detected by the stroke detection means at a moment when the brake system operation determination means determine the operation of the brake system, and PA1 means for storing an initial value of the master cylinder pressure value detected by the master cylinder pressure value detection means at the moment when the brake system operation determination means determine the operation of the brake system, PA1 wherein the brake system operation determination means determine the operation of the brake system when the first judgment means judge the first positive braking intention of the driver, while the third judgment means judge the third positive braking intention of the driver, even when the second judgment means do not judge the second positive braking intention of the driver, and PA1 wherein the control means control a pressure of the brake fluid supplied to the selected one of the wheel cylinders such that the vehicle is more decelerated along with increase of the depression stroke of the brake pedal above the initial stroke value and increase of the master cylinder pressure above the initial mater cylinder pressure value by laying the first and second weights on the increase of the depression stroke and the increase of the master cylinder pressure, respectively, the first weight being decreased while the second weight being increased relative to one another along with increase of the braking applied to the vehicle.
The last paragraph of the above-mentioned former invention means, in more detail, that the brake fluid pressure determination means determine the brake fluid pressure supplied to the selected one or ones of the wheel cylinders to accomplish a deceleration Gt which is a weighted sum of Gst and Gpt such as EQU Gt=(1-.alpha.)Gst+.alpha.Gp,
wherein Gst is a target deceleration scheduled to increase along with increase of depression stroke Sp of the brake pedal as shown in FIG. 9, while Gpt is a target deceleration scheduled to increase along with increase of master cylinder pressure Pm as shown in FIG. 10, and the weighting factor .alpha. is such a value which is scheduled to change along with the total deceleration Gt as shown in FIG. 11.
By the pressure of the brake fluid supplied to the wheel cylinders being determined according to a weighted combination of a first braking effect corresponding to the brake pedal depression stroke and a second braking effect corresponding to the master cylinder pressure so that the weight is shifted from the first braking effect to the second braking effect along with increase of the total target deceleration or the total braking force, the initial insensibleness of the brake control based upon the detection of the master cylinder pressure is compensated for by the high sensibleness of the brake control based upon the brake pedal depression stroke, while such a compensation is gradually cancelled according to increase of the braking force out of the initial insensible region, so that a desirable linearity is obtained in the relationship between the braking force applied to the wheels and the reaction force perceived by the driver against the depression of the brake pedal.